Method for manufacturing hard and compact protective layers on magnesium and magnesium alloys



Patented Dec. 22, 1942 METHOD FOR MANUFACTURING HARD AND COMPACT PROTECTIVE LAYERS N MAG- NESIUM AND MAGNESIUM ALLOYS Nikolai Budiloif, Berlin-Friedenau, and Willy Schnabel, Berlin Charlottenburg, Germany; vested in the Alien Property Custodian No Drawing. Application November 16, 1938, Se-

rial No. 240,850. In Germany December 1,

4 Claims.

This invention relates to a method for manufacturing hard and compact protective coatings on magnesium and magnesium alloys.

It is known to strengthen the oxide layers usually present on magnesium by a chemical or electrochemical method requiring the immersion of the objects to be oxidized, in a suitable solution.

By the known electrochemical methods for oxidizing magnesium or magnesium alloys, only relatively thin coatings can be produced. It is also known to produce adhesive colored and protective coatings on zinc and on alloys consisting essentially of zinc, by anodically treating the objects to be coated in an alkaline reactive solution containing additions of compounds of amphoteric metals. The coatings produced by this method on objects of zinc or zinc alloys are also very thin.

It has been proposed to employ as electrolytes solutions of alkali aluminates or alkaline earth aluminates for the production of hard and compact oxide layers on metals and alloys. If metallic objects of iron, nickel, copper, zinc, magnesium, brass, German silver and the like are treated according to this method, thick and fastadhering coatings of mixed oxides having good insulating properties and high absorptive capacity are produced.

It has proved disadvantageous for many purmetal only to a slight extent against corrosive influences. The high porosity of the oxide coatings on magnesium is due to the fact that when converting magnesium into its oxide, a reduction in volume occurs so that the coatings produced are apt to develop a great number of pores and flaws. Due to their small hardness, the coatings thus produced are also rather unsuitable for sufilciently protecting the metallic substratum against mechanical influences. The surface portions of the substratum uncovered by mechanical injuries or flaws and pores are usually the starting points of corrosive attacks.

It is an object of the invention to produce protective coatings on magnesium and magnesium alloys which are free from the abovementioned drawbacks.

Another more particular object of the invention is to provide objects of magnesium or magnesium alloys with compact oxide coatings of great hardness and uniformity. Other objects will become apparent from the following description.

According to the invention, objects consisting of magnesium or its alloys are treated under the action of an electric current in a solution of alkali salts or alkaline earth metal salt of an amphoteric metal, for instance in an aluminate electrolyte, containing additions of compounds whose anions form compounds with magnesium which are insoluble or hardly soluble.

Additions of the above character are, for instance, sodium fluoride, sodium phosphate and sodium arsenate. These substances are suitable for producing protective coatings of considerable hardness and of slight porosity, since they are produced in an insoluble or hardly soluble form while their volume increases materially, thus preventing the formation of pores to a great extent.

The electrolysis according to the invention is preferably effected at normal or only slightly elevated temperatures, for instance 15 to 30 C., and with a current density of 1 to 3 amps. per square decimeter.

As compared with pure oxide coatings, the protective coatings produced according to the method of the invention have, besides the abovementioned advantageous properties, great adhesive force and a uniform structure so that they constitute a protection against corrosion hitherto unattainable on objects of magnesium and its alloys. Particularly striking are their electrical insulating effect and their extraordinary hardness which render the protective coatings particularly suitable for parts subjected to friction. In some cases the hardness of these coatings is greater than that of the oxide coating obtained by the electrolytic oxidation of aluminum.

The hardness of the coatings produced ac cording to the invention, as determined by tests performed with a cone point, was 5 to 10 times greater than the hardness of reference test pieces coated according to the prior art, for instance in sodium alkaline solutions. Wear tests of specimens according to the invention, performed with the aid of the Siemens wear tester, required at the tested point several millions of double strokes under a load of 300 grams to wear through the coating.

The method according to the invention may be carried out in different ways. Thus, for instance, it is possible to employ alternating current, direct current, direct current upon which alternating current is superimposed, or the like. The coatings may be subsequently treated in a known manner. For instance, they may be densified or colored. They also constitute a particularly suitable support for paints or varnishes. Coatings according to the invention also reduce the danger of ignition and combustion of the magnesium metal. The protective coatings produced according to the invention may be advantageously employed as supports for light-sensitive substances so that they may be employed in a known manner for the production of images, patterns and other characters.

Example 1. The method of producing hard and com pact protective coatings on objects of magnesium and magnesium alloys, which comprises subjecting the objectsto be coated to electrolysis in a bath consisting substantially of an aluminate electrolyte and an addition selected from the group consisting of the soluble phosphates, arsenates, and fluorides of the alkali metals.

2. The method of producing hard and compact protective coatings on objects of magnesium and magnesium alloys, which comprises subjecting the objects to be coated to electrolysis in a bath consisting substantially of an aluminate electrolyte and an addition selected from the group consisting of the soluble phosphates, arsenates, and fluorides of the alkali metals, maintaining said solution at a temperature between about 15 and about 30 centigrade, and maintaining .a current density during the electrolysis between about 1 and about 3 amperes per square decimeter.

3. The method of producing hard and compact protective coatings on objects of magnesium and magnesium alloys, which comprises subjecting the objects to be coated to electrolysis by alternating current in a bath consisting substantially of a solution of about 5% sodium aluminate, about 3% sodium fluoride and about 5% trisodium phosphate.

4. The method of producing hard and compact protective coatings on objects of magnesium and magnesium alloys, which comprises subjecting the objects to be coated to electrolysis in a bath consisting substantially of an aluminate electrolyte and an addition selected from the group consisting of the soluble phosphates, arsenates, and fluorides of the alkali metals, and maintaining said bath at a temperature'between about 15 and about 30 centigrade.

NIKOLAI BUDILOFF. WILLY SCHNABEL. 

